As is known in the art, powder metal gamma prime (.gamma.') precipitation strengthened nickel-base superalloys are capable of providing a good balance of creep, tensile and fatigue crack growth properties to meet the performance requirements of components used in gas turbine engines. Typically, such components are produced by some form of consolidation, such as extrusion consolidation, then isothermally forged to the desired outline, and finally heat treated. These processing steps are designed to retain a particular grain size within the component.
In order to improve the fatigue crack growth resistance and mechanical properties of these materials at elevated temperatures, these alloys are heat treated above the .gamma.' solvus temperature (generally referred to as a supersolvus heat treatment), to cause significant, uniform coarsening of the grains, resulting in a grain size of as large as about ASTM 6. (Reference throughout to ASTM grain sizes is in accordance with the standard scale established by the American Society for Testing and Materials.) The term "uniform" with respect to grain growth means the substantial absence of non-uniform critical grain growth. Critical grain growth is defined as localized abnormal excessive grain growth to grain diameters exceeding a desired range, causing a detrimental effect on mechanical properties such as tensile and fatigue.
To meet the increasing demand for higher temperature capabilities for turbine disks used in gas turbine engines, it is necessary to achieve coarser grains in the rim portion of the disk where the operating temperature of the disk is highest, while finer grains are desired near the center bore of the disk in order to yield greater hardness and strength. Engineering estimates have indicated a substantial improvement in creep capability for a disk with coarse grains at its rim. For example, at a stress level of about 70,000 pounds per square inch, a superalloy material having a grain size of about ASTM 2 is estimated to provide an approximately 100.degree. F. higher temperature capability than that possible with the same material having a grain size of about ASTM 6 when subjected to a 200 hour creep test to 0.2 inch. However, current practices have been unable to produce uniform grain sizes of coarser than about ASTM 5 in superalloy articles formed by powder metallurgy.
A thermomechanical process disclosed in U.S. Pat. No. 4,957,567 to Krueger et al., assigned to the assignee of this invention, discloses the production of uniform grain sizes ranging from ASTM 2-9. In practice, the process taught by Krueger et al. is employed to produce components with average grain sizes in the range of about ASTM 6 through 9, in that the process is less reliable in producing grain sizes in the range of ASTM 2 through 5.
Testing reported by J. M. Hyzak et al. at the Proceedings of the Seventh International Symposium on Superalloys, The Minerals, Metals & Materials Society, 1992, has suggested that grain coarsening of UDIMET 720 can be achieved during forging at temperatures near or above the .gamma.' solvus temperature of the material. However, deformation by this technique is not superplastic, such that the extent of deformation is significantly limited. A propensity for grain boundary cracking has also been identified as a potential limitation of this process. Furthermore, possible uniform grain coarsening was not documented, and a propensity for critical grain growth exists with the Hyzak et al. process, which would result in an unacceptable forged product. Also, the method taught by J. M. Hyzak et al. is not amenable to high resolution sonic inspections due to the presence of as-forged coarse grains, nor is the method amenable to post forge supersolvus heat treatment due to the high risk of abnormal grain growth. Finally, the method taught by Hyzak et al. is generally incompatible with methods for producing dual alloy disks in that the preforms would not be fine grained and, therefore, superplastic deformation to achieve desired high strains at the bondline would not be possible.
Dual alloy disks and differentially heat treated monolithic disks known in the prior art tend to have dual microstructures, such that the bulk of the rim is one grain size and the bore is a uniform finer grain size. While the resulting dual property condition is an improvement over conventional monolithic disks, such improvements are still limited to a maximum grain size of about ASTM 6. Ideally, the grain size and microstructure of a disk should vary radially in keeping with the temperature and stress-state gradient experienced during operation, such as a grain size of at least about ASTM 5 and preferably coarser at the disk rim, and a grain size of about ASTM 10 at the disk bore. In addition, to achieve a suitable balance of mechanical properties such as burst strength, creep, low cycle fatigue, notch ductility and damage tolerance, accurate control of grain size is required.
Accordingly, what is needed is a process by which an article can be formed from a .gamma.' precipitation strengthened nickel-base superalloy such that at least a portion of the article is characterized by a uniform grain size of at least about ASTM 5. Furthermore, it would be desirable if such a process achieved a grain size and microstructure which can be controlled to vary uniformly between portions of the article in accordance with the temperature and stress-state gradient experienced during operation of the article.